Stopping an old machlokes is a delicate matter because often each party feels that the other party is guilty of initiating the machlokes. Therefore, we suggest that you make a cheshbon hanefesh. Carefully check your deeds. You might find some fault in your behavior towards the other person.
It is never too late to do teshuvah. It seems from the Gemara (Sanhedrin 110.) that the children of Korach did teshuvah while the earth was swallowing them. Therefore, "A place rose in Gehinom for them, and they sat there and sang songs of gratitude to Hashem."
The Trisker Magid zt'l says that we say a chapter of Tehillim (47, ԀԀԀԀԀ ԀԀԀԀ ԀԀԀ ԀԀԀԀԀ) before we blow the shofar to remind us of the power of teshuvah.
Rebbe Yehoshua of Belz zt'l explains that Moshe wanted them alive so they would have another opportunity to do teshuvah. Indeed, Korach's children grabbed the opportunity and were saved.